Previous methods for making multi-mode waveguide circuits have used photo-lithography and relatively thick layers of a photoresist to create the circuit pattern. The advantage of lithographic methods is that they are amenable to mass production and can provide good control of circuit geometry and dimensions. Two significant disadvantages are:
1) Lithographic methods are limited to producing only square or rectangular channels; this results in significant light loss due to the geometric mis-match between a round fiber and a square channel. PA1 2) Lithographic methods inherently produce sidewall striations (non-smooth channels) due to mask imperfections, photoresist impurities, and various sources of scattered light in the lithographic process. These striations in the photoresist image introduce scattering loss in the resulting optical circuits as light is guided down the channel.
In the cases where the waveguide itself is composed of a photoresist or a photosensitive material, there is the added disadvantage that such materials are not optimized for light transmission and may exhibit a high loss due to absorption of the light as it propagates through the waveguide. In general, lithographic processes work well for thin photoresist layers, but are difficult to apply in the thick films required for multi-mode optical circuits.
Another method for making multi-mode optical circuits involves twisting glass or plastic fibers together, and then heating/stretching the twisted region so as to form N.times.N couplers/splitters. In some cases, this method works well, but there are significant disadvantages: 1) these fused devices are typically made one at a time and are not amenable to mass production; they are relatively expensive; 2) it is difficult to control the splitting ratios or uniformity with fused fiber devices; and 3) fused fiber devices are limited in that some types of circuit functions cannot be implemented by fused fiber devices.